Add regression refresher

[cjuracek]

Some initial ideas:

• Formulas for both single + matrix multivariate linear regression
• OLS estimators, derivation (matrix), and properties (bias = 0 (unbiased), var = sigma^2 (X^TX)^-1)
• Assumptions of model
• Interactions
• Interpretation of beta coefficients
• Interpreting R output
• Lasso + Ridge regression
• Mixed effect models (random effects)
• Model selection for both fixed + random effects and derivations test statistics associated with them (likelihood ratio / F-test)